This invention relates to the production of semiconductor devices and is concerned with a gold electroplating solution for use in plating the nickel-iron substrates generally employed in such devices and with a method of electrodepositing gold on such substrates using the gold electroplating solution.
Semi-conductor devices normally comprise a germanium or silicon chip or slice supported by a substrate which, in the case of an integrated circuit flat pack, is generally termed a lead frame. Headers and lead frames are generally formed from nickel-iron alloys and are electrolytically gold plated to a thickness of about 2 microns so as to permit the bonding of the chip or slice to the substrate by formation of a gold-silicon or gold-germanium eutectic, since both silicon and germanium form low melting point alloys with gold. The gold-silicon eutectic has a melting point of 370.degree. C. and contains approximately 3 percent silicon. The bonding operation, known as die-attachment or die-bonding, is normally completed in a few seconds at temperatures up to 500.degree. C. However, the conventional test for gold deposits on nickel-iron substrates is a requirement to withstand a temperature of 500.degree.-600.degree. C. for five minutes in air without tarnishing or blistering of the gold deposit.
The gold deposit of 2 microns on the nickel-iron alloy serves the dual purpose of producing a pore-free film as well as supplying the gold for the resultant eutectic at the die-bonding stage. A porous gold deposit would allow the iron from the substrate to diffuse through to the surface forming an oxide which would discolor the component as well as inhibit the eutectic formation.
Lead frames can be selectively gold plated (i.e., plated only over desired areas), one of the main methods being by the use of a mechanical masking tool in conjunction with a high velocity jet of a gold electrolyte. However, it has hitherto been found that, due to the gold electrolytes presently used becoming contaminated, principally with iron and nickel, a marked deterioration in performance occurs progressively with increasing age of the electrolyte. The substrate nickel-iron dissolves chemically in the presently used gold electrolytes as a result of being attacked by the citrates, pyrophosphates and oxalates present in the electrolyte as chelating and buffering agents. The dissolution of nickel and iron into these electrolytes is related to contact time. As the majority of the semi-conductor devices are selectively gold plated using high current densitites of the order of 8 A/dm.sup.2, the nickel-iron contaminant in the electrolyte affects the physical characteristics and structure of the gold deposit.